Method for clearing native oxide

ABSTRACT

A method for clearing native oxide is described. A substrate is provided, including an exposed portion whereon a native oxide layer has been formed. A clearing process is performed to the substrate using nitrogen trifluoride (NF 3 ) and ammonia (NH 3 ) as a reactant gas, wherein the volumetric flow rate of NF 3  is greater than that of NH 3 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims the benefitof U.S. patent application Ser. No. 12/129,978, filed May 30, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a semiconductor process, and in particular, toa method for clearing native oxide.

2. Description of the Prior Art

Along with rapid progress of semiconductor technology, the dimensions ofsemiconductor devices are reduced and the integrity thereof promotedcontinuously to further advance the operating speed and performance ofthe integrated circuit. As the demand for device integrity is raised,the changes in physical properties, such as contact resistance betweenthe devices, have to be considered to avoid a great impact on theoperating speed and performance of the circuit.

Taking the formation of a contact plug as an example, after a contacthole is formed but before the contact hole is filled with conductivematerial, a clearing process is generally conducted to clear theimpurities or native oxide at the bottom of the contact hole so as toreduce the contact resistance. Nevertheless, there are still someproblems in the foregoing method, so that the performance of the circuitis reduced.

FIGS. 1A-1B are schematic cross-sectional views illustrating aconventional fabrication process of a contact plug. As shown in FIG. 1A,a dielectric layer 102 has been formed on a silicon substrate 100, and acontact hole 104 has been formed in the dielectric layer 102 exposing apartial surface of the silicon substrate 100. As the exposed surface ofthe silicon substrate 100 contacts with the atmosphere, an oxidationtakes place so that a native oxide layer 106 is formed at the bottom ofthe contact hole 104. A conventional method removes the native oxidelayer 106 in a physical manner with argon (Ar) sputtering to solve theproblems arising form the native oxide layer 106. However, the removalperformance of the native oxide layer 106 by means of Ar sputtering isnot satisfactory due to the high aspect ratio of the contact hole 104formed in current fabrication processes.

As shown in FIG. 1B, a facet 110 is formed at the sidewall of thecontact hole 104 during the Ar sputtering because of the arrival angleof Ar ion. While the contact hole 104 is filled with a conductive layer108 in the later process to accomplish the contact plug, the conductivelayer 108 formed between adjacent contact holes 104 tends to bring abridging 112 due to the facet 110. Accordingly, the electricalproperties of the devices are subjected to serious impact. Moreover, asthe dimensions of the devices are miniaturized, by-products producedfrom the sputtering process are easily re-deposited in the contact hole104, so that the profiles and critical dimensions of the contact hole104 are changed.

As a result, how to effectively clear native oxide and also ensure thequality of the later-formed devices to improve the process reliabilityand device performance is one of the immediate issues to be solved inthe art.

SUMMARY OF THE INVENTION

Accordingly, this invention is directed to a method for clearing nativeoxide so as to prevent the profiles and critical dimensions of openingsfrom being changed and further reduce the contact resistance.

The method for clearing native oxide of this invention is described asfollows. A substrate is provided, including an exposed portion on whicha native oxide layer has been formed. A clearing process is performed tothe substrate using nitrogen trifluoride (NF₃) and ammonia (NH₃) as areactant gas, wherein the volumetric flow rate of NF₃ is greater thanthat of NH₃.

According to an embodiment of this invention, the volumetric flow rateratio of NF₃ to NH₃ is within the range of 1.5:1 to 5:1, possibly being2:1.

According to an embodiment of this invention, the reactant gas causesNH_(x)HF_(y) to form in the clearing process, where x and y both are notzero.

According to an embodiment of this invention, the RF power applied inthe clearing process is within the range of 5 W to 200 W, possibly being60 W.

According to an embodiment of this invention, the duration of theclearing process is within the range of 5 seconds to 100 seconds.

According to an embodiment of this invention, the clearing process isperformed at a temperature below 100° C.

According to an embodiment of this invention, the method furthercomprises performing a first annealing process after the clearingprocess. In such a case, the first annealing process may be performed ata temperature within the range of 360-440° C. The duration of the firstannealing process is within the range of 40-80 seconds.

According to an embodiment of this invention, the method furthercomprises performing a second annealing process after the clearingprocess but before the first annealing process. In such a case, thesecond annealing process maybe an in-situ annealing process, and may beperformed at a temperature over 100° C.

According to an embodiment of this invention, the method furthercomprises adding a diluting gas into the reactant gas. The diluting gasmay be an inert gas, and the volumetric flow rate ratio of the dilutinggas to NF₃ to NH₃ is within the range of 10:1:1 to 5:1:5.

According to an embodiment of this invention, the exposed portioncomprises a silicon-containing material or a metal. Thesilicon-containing material may be monocrystalline silicon, polysiliconor metal silicide.

According to an embodiment of this invention, the exposed portion is aregion exposed in a contact hole, in a damascene opening, or in a trenchof a shallow trench isolation (STI) structure to be formed.

As mentioned above, the method for clearing native oxide of thisinvention adjusts the mixing ratio of the reactant gas to have thevolumetric flow rate of NF₃ greater than that of NH₃, so that the nativeoxide on the surface of the exposed portion is etched by the productformed from NF₃ and NH₃ making the etching reaction easily saturated.Consequently, the profiles and critical dimensions of the openings canbe prevented from being changed, and also the device performance can beenhanced.

In order to make the aforementioned and other features and advantages ofthis invention more comprehensible, preferred embodiments accompaniedwith figures are described in detail below.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment that isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are schematic cross-sectional views illustrating aconventional fabrication process of a contact plug.

FIG. 2 is a flowchart illustrating a method for clearing native oxideaccording to an embodiment of this invention.

FIGS. 3A-3B are schematic cross-sectional views illustrating afabrication process of a contact plug according to an embodiment of thisinvention.

FIGS. 4A-4B are schematic cross-sectional views illustrating afabrication process of a STI structure according to an embodiment ofthis invention.

FIGS. 5A-5B are schematic cross-sectional views illustrating afabrication process of a dual damascene structure according to anembodiment of this invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

FIG. 2 is a flowchart illustrating a method for clearing native oxideaccording to an embodiment of this invention.

Referring to FIG. 2, in step S200, a substrate is provided, which has anexposed portion whereon a native oxide layer has been formed. Theexposed portion can be a portion of a monocrystalline silicon substrateor a multi-film structure, wherein each film may include dopedpolysilicon, metal silicide or metal. The exposed portion is, forexample, exposed in a contact hole, in a damascene opening or in atrench of a STI structure to be formed.

Afterwards, in step S210, a clearing process is performed to thesubstrate to remove the native oxide layer with nitrogen trifluoride(NF₃) and ammonia (NH₃) as a reactant gas, wherein the volumetric flowrate of NF₃ is greater than that of NH₃. In an embodiment, thevolumetric flow rate ratio of NF₃ to NH₃ is within the range of 1.5:1 to5:1. In another embodiment, the volumetric flow rate ratio of NF₃ to NH₃is 2:1.

Moreover, an inert gas can be introduced into the reactant gas as adiluting gas or a carrier gas during the clearing process. In anembodiment, the introduced inert gas is argon (Ar) gas or helium (He)gas. The RF power applied in the clearing process is within the range of5 W to 200 W. In an embodiment, the clearing process is performed at atemperature below 100° C. In another embodiment, the clearing process isperformed at a temperature below 50° C. The pressure set in the clearingprocess is maintained at about 5 Torr. In addition, the duration of theclearing process is usually within the range of 5 seconds to 100seconds, depending on the thickness of the native oxide layer.

NF₃ and NH₃ each can be dissociated into reactive species by RF power,as represented by the following reaction formulae:

NF₃→NF_(x)+F+N

NH₃→NH_(x)+H+N.

The fluorine dissociated from NF₃ and the hydrogen dissociated form NH₃re-combine to form hydrogen fluoride (HF). HF can further combine withNH_(x) dissociated from NH₃ or, in the alternative, with a small amountof NH₃ to form NH_(x)HF_(y), where x and y both are not zero. Highlyreactive NH_(x)HF_(y) derived from NF₃ and NH₃ removes the native oxidelayer by means of etching. In an embodiment, the native oxide layerincludes silicon dioxide (SiO₂) , which reacts with NH_(x)HF_(y) to forma solid product, ammonium hexafluorosilicate ((NH₄)₂SiF₆), asrepresented by the following reaction formula:

NH_(x)HF_(y)+SiO₂→(NH₄)₂SiF₆.

In next step S220, an in-situ annealing process is performed. In otherwords, the substrate is heated at the same place after the step S210.The in-situ annealing process is performed at a temperature over 100° C.and the duration thereof about 60 seconds, for example.

In an embodiment, the native oxide layer includes SiO₂, and the solidproduct (NH₄)₂SiF₆ generated in the step S210 is decomposed intovolatile gas and removed by means of heating in the in-situ annealingprocess, as represented by the following reaction formula:

(NH₄)₂SiF₆→SiF₄+NH₃.

It is noted that this invention adjusts the mixing ratio of the reactantgas so that the amount of NF₃ is greater than that of NH₃ and applieshigher RF power to generate the plasma. Since a smaller amount of NH_(x)is formed due to the reduced provision of NH₃ to decrease the amount ofNH_(x)HF_(y) generated form the reactant gas and the reaction ofNH_(x)HF_(y) with the oxide is diffusion-controlled, the reaction ratein removing the native oxide is lowered. After entire NH_(x)HF_(y)derived from NF₃ and NH₃ reacts with the native oxide layer, a thinlayer of the product is formed on the surface of the exposed portion,and thus the reaction is saturated preventing over-removal effectively.

In an embodiment, the cycle of the steps S210 to S220 can be repeated atleast one time after the step S220 so as to clear the native oxide layercompletely.

After the step S220, another annealing process may be performedoptionally to re-crystallize the silicon substrate or metal silicide(step S230). Thereby, the defect density within the silicon substrate ormetal silicide is decreased, and the material structures and electricalproperties of the surface are repaired. The annealing process in thestep 5230 is performed at a temperature within the range of 360-440° C.,and the duration thereof is within the range of 40-80 seconds. In anembodiment, the annealing process in the step S230 is performed at about400° C. for about 60 seconds.

In the field of semiconductor process, several practical applications ofthe method for clearing native oxide according to this invention areprovided below. It is to be understood that the following manufacturingprocedures are intended to explain the sequence of the steps of thepresent method for clearing native oxide in practical applications andthereby enable those of ordinary skill in the art to practice thisinvention, but are not intended to limit the scope of this invention. Itis to be appreciated by those of ordinary skill in the art that otherelements, such as the substrate, the gate structure, the doping regions,the conductive lines and the openings, can be arranged and formed in amanner not shown in the illustrated embodiments according to knownknowledges in the art.

FIGS. 3A-3B are schematic cross-sectional views illustrating thefabrication process of a contact plug according to an embodiment of thisinvention.

Referring to FIG. 3A, a substrate 300 is provided, which may be asemiconductor substrate, e.g., a silicon substrate. A dielectric layer304 is formed on the substrate 300, and an opening 310 which exposes aregion 302 of the substrate 300 is formed in the dielectric layer 304.The exposed portion 302 is, for example, a conductive portion containingsilicon. The silicon-containing conductive portion is, for example,agate electrode or a doped region. The material of the gate can be dopedpolysilicon. The doped region is, for example, a source region or adrain region, and the material thereof can be N-type or P-type dopedmonocrystalline silicon or polysilicon. In an embodiment, thesilicon-containing conductive region maybe a gate or a doped regionwhereon a metal silicide layer has been formed. The material of themetal silicide layer can be silicide of a refractory metal, and therefractory metal is, for example, nickel (Ni) , cobalt (Co) , titanium(Ti) , copper (Cu) , molybdenum (Mo) , tantalum (Ta) , tungsten (W) ,erbium (Er) , zirconium (Zr) , platinum (Pt) , or one of the alloys ofthe foregoing metals. The material of the dielectric layer 304 can besilicon dioxide, borophosphosilicate glass (BPSG) , phosphosilicateglass (PSG) or any other suitable dielectric material. In thisembodiment, a MOS transistor including a gate structure 330 and sourceand drain regions 332 is taken as an example, and the exposed portion302 exposed in the opening 310 in the dielectric layer 304 is the sourceand drain regions 332. In another embodiment, the opening 310 is formedover the gate structure 330. That is, the exposed portion 302 is thegate conductor of the gate structure 330.

After the opening 310 is formed, wafers might be briefly exposed to theatmosphere while being transferred to the next processing equipment forconducting the subsequent process. As shown in FIG. 3A, since theformation of the opening 310 makes the surface of the exposed portion302 contact with oxygen in the atmosphere, oxidation takes place on thesurface of the exposed portion 302 at the bottom of the opening 310 andin consequence a native oxide layer 320 is formed.

Referring to FIG. 3B, a clearing step S300 that uses the method forclearing native oxide of this invention is done to remove the nativeoxide layer 320, possibly in accordance with the procedure shown in FIG.2. In an embodiment, the gas introduced in the clearing process includesAr, NF₃ and NH₃, and the ratio of the volumetric flow rates of the threegases is within the range of 10:1:1 to 5:1:5. The RF power applied inthe clearing process is within the range of 5 W to 200 W, such as 60 W.In an embodiment, the clearing process is performed at a temperaturebelow 100° C. In another embodiment, the clearing process is performedat a temperature below 50° C. In addition, the pressure set in theclearing process is maintained at about 5 Torr. The duration of theclearing process depends on the thickness of the native oxide layer 320and is usually within the range of 5 seconds to 100 seconds, such as 40seconds.

Afterwards, an adhesion layer 306 is formed conformally on the surfaceof the opening 310 to enhance adhesion between the dielectric layer 304and the conductive material which fills the opening 310 in thesubsequent process. The material of the adhesion layer 306 is, forexample, a refractory metal or a nitride or an alloy thereof, such astitanium, titanium nitride, tungsten, tungsten nitride,titanium-tungsten alloy, tantalum, tantalum nitride, nickel ornickel-vanadium alloy. A conductive layer 308 is then filled in theopening 310 to complete the fabrication of the contact plug. Thematerial of the conductive layer 308 is, for example, doped polysilicon,aluminium, tungsten or copper.

It is noted that the opening 310 and the adhesion layer 306 may beformed in different process equipments or be formed not right after theformation of the opening 310. Therefore, not only after the formation ofthe opening 310 but also before the formation of the adhesion layer 306,a process for clearing native oxide including the steps S210 to S220 inFIG. 2 can be performed. As a result, the native oxide layer 320 can beremoved completely, and the contact resistance of the contact plug canbe reduced to promote the device performance.

FIGS. 4A-4B are schematic cross-sectional views illustrating thefabrication process of a STI structure according to an embodiment ofthis invention.

Referring to FIG. 4A, a substrate 400 like a silicon substrate isprovided. A patterned pad layer 412 and a patterned mask layer 414 areformed sequentially on the substrate 400. The material of the patternedpad layer 412 is silicon dioxide and the material of the patterned masklayer 414 is silicon nitride, for example. Trenches 410 are then formedin the substrate 400 by removing the partial exposed substrate 400 usingthe patterned mask layer 414 as the mask. Since the material of thesubstrate 400 is monocrystalline silicon, oxidation takes place on theexposed surface of the substrate 400 in the trenches 410 to form anative oxide layer 420 when the substrate 400 contacts with oxygen inthe atmosphere.

Referring to FIG. 4B, a clearing step 5400 that utilizes the method forclearing native oxide of this invention as illustrated in FIG. 2 isconducted to remove the native oxide layer 420, so as to ensure the STIstructure a good quality. In an embodiment, the gas introduced in theclearing process includes Ar, NF₃ and NH₃, and the volumetric flow rateratio of the gases is within the range of 10:1:1 to 5:1:5. The RF powerapplied in the clearing process is within the range of 5 W to 200 W. Theclearing process is performed at a temperature below 100° C. Thepressure suitably set in the clearing process is maintained at about 5Torr. The duration of the clearing process is usually within the rangeof 5 seconds to 100 seconds, depending on the thickness of the nativeoxide layer 420.

A liner 402 is then formed conformally on the surfaces of the trenches410. The material of the liner 402 is, for example, silicon dioxide, andthe formation method thereof is thermal oxidation. An insulating layer(not shown) is formed on the substrate 400 filling up the trenches 410.A portion of the insulating layer is then removed to planarize thesurface thereof by means of chemical mechanical polish (CMP) or etchingback using the patterned mask layer 414 as the stop layer, so thatisolation structures 404 are formed. Subsequently, the patterned masklayer 414 and the patterned pad layer 412 are removed. The material ofthe insulating layer is, for example, silicon dioxide, and the formationmethod thereof is chemical vapor deposition (CVD).

FIGS. 5A-5B are schematic cross-sectional views illustrating thefabrication process of a dual damascene structure according to anembodiment of this invention.

Referring to FIG. 5A, a substrate is provided, whereon a metal layer 502and dielectric layers 504 and 506 have been formed in sequence. Thematerial of the metal layer 502 is, for example, copper, aluminium ortitanium. The material of the dielectric layers 504 and 506 is, forexample, silicon dioxide BPSG, PSG or any other suitable dielectricmaterial. An opening 510 is then formed in the dielectric layers 504 and506 so that a partial surface of the metal layer 502 is exposed. In anembodiment, the opening 510 is a dual damascene opening composed of atrench 510 b and a via hole 510 a for the formation of the dualdamascene structure in the subsequent process. The method for formingthe opening 510 is, for example, removing a portion of the dielectriclayers 504 and 506 to form the via hole 510 a in the dielectric layer504 and the trench 510 a in the dielectric layer 506.

Since the partial surface of the metal layer 502 exposed at the bottomof the opening 510 is oxidized for the contact with air, a native oxidelayer 520 is formed on the surface of the metal layer 502 at the bottomof the opening 510.

Referring to FIG. 5B, likewise, a clearing step 5500 utilizing themethod for clearing native oxide of this invention as illustrated inFIG. 2 is conducted to remove the native oxide layer 520. In anembodiment, the gas introduced in the clearing process includes Ar, NF₃and NH₃, and the volumetric flow rate ratio of the gases is within therange of 10:1:1 to 5:1:5. The RF power applied in the clearing processis within the range of 5 W to 200 W. The clearing is performed at atemperature below 100° C. The pressure suitably set in the clearingprocess is maintained at about 5 Torr. The duration of the clearingprocess is usually within the range of 5 seconds to 100 seconds,depending on the thickness of the native oxide layer 520.

Afterwards, a barrier layer 508 is formed conformally on the surface ofthe opening 510. The material of the barrier layer 508 is, for example,a refractory metal or a nitride or an alloy thereof, such as titanium,titanium nitride, tungsten, tungsten nitride, titanium-tungsten alloy,tantalum, tantalum nitride, nickel, nickel-vanadium alloy. A seed layer(not shown) is then formed on the surface of the opening 510 to furtherenhance adhesion of the subsequently-formed metal layer. A metal layer512 is filled in the opening 510 to form a dual damascene structure. Thematerial of the metal layer 512 is, for example, copper, aluminium ortungsten. It is noted that after the barrier layer 508 is formed on thesurface of the opening 510 but before the seed layer is formed, anative-oxide clearing step S500 including the steps S210 to S220 shownin FIG. 2 can be conducted again, so as to ensure the quality of thebarrier layer 508 and reduce the contact resistance of the damascenestructure.

Moreover, though this embodiment takes the formation of a dual damascenestructure as an example, it does not limit the scope of this invention.The method of this invention can also be applied to a damascene processwhich forms a metal via plug or a conductive line only, wherein thenative-oxide clearing process as described in the above-mentionedembodiment can be utilized to remove the native oxide layer formed onthe metal layer after the via hole or the trench is formed, or before orafter the barrier layer is formed. Certainly, in other embodiment, anative oxide layer formed on a surface of any opening, hole or trenchwith a high aspect ratio can be removed by using the method of thisinvention. Other applications and modifications should be apparent tothose of ordinary skill in the art according to the above-mentionedembodiments, and hence, a detailed description thereof is omittedherein.

In view of the above, the method for clearing native oxide of thisinvention uses a reactant gas including a larger proportion of NF₃ and asmaller proportion of NH₃ with a higher RF power to form NH_(x)HF_(y)for removing the native oxide layer formed in the exposed portion afterthe opening is formed or before the liner, the adhesion layer or thebarrier layer is formed. Since the amount of NH_(x)HF_(y) generated fromthe reactant gas is small and the reaction of NH_(x)HF_(y) with theoxide is diffusion-controlled, the reaction rate in removing the nativeoxide layer can be lowered. Accordingly, after entire NH_(x)HF_(y)reacts with the native oxide layer, the reaction with the native oxidelayer is saturated, so that over-removal is prevented and the profilesand critical dimensions of openings are prevented from being changed.Thus, the quality of the structure formed subsequently in the openingcan be ensured, and the process reliability can also be promotedremarkably.

This invention has been disclosed above in the embodiments, but is notlimited to those. It is known to persons skilled in the art that somemodifications and innovations may be made without departing from thespirit and scope of this invention. Hence, the scope of this inventionshould be defined by the following claims.

Those skilled in the art will readily observe that numerousmodifications and alterations of the device and method may be made whileretaining the teachings of the invention. Accordingly, the abovedisclosure should be construed as limited only by the metes and boundsof the appended claims.

1. A method for clearing native oxide, comprising: providing amonocrystalline silicon substrate, wherein the monocrystalline siliconsubstrate has an exposed portion and a native oxide layer has beenformed on the exposed portion; and performing a clearing process to themonocrystalline silicon substrate using nitrogen trifluoride (NF₃) andammonia (NH₃) as a reactant gas.
 2. The method according to claim 1,wherein the reactant gas forms NH_(x)HF_(y) in the clearing process, andx and y both are not zero.
 3. The method according to claim 1, whereinan RF power applied in the clearing process is within a range of 5 W to200 W.
 4. The method according to claim 3, wherein the RF power appliedin the clearing process is 60 W.
 5. The method according to claim 1,wherein a duration of the clearing process is within a range of 5seconds to 100 seconds.
 6. The method according to claim 1, wherein theclearing process is performed at a temperature below 100° C.
 7. Themethod according to claim 1, further comprising performing a firstannealing process after the clearing process.
 8. The method according toclaim 7, wherein a temperature of the first annealing process is withina range of 360° C. to 440° C.
 9. The method according to claim 7,wherein a duration of the first annealing process is within a range of40 seconds to 80 seconds.
 10. The method according to claim 7, furthercomprising performing a second annealing process after the clearingprocess but before the first annealing process.
 11. The method accordingto claim 10, wherein the second annealing process is an in-situannealing process.
 12. The method according to claim 10, wherein atemperature of the second annealing process is over 100° C.
 13. Themethod according to claim 1, further comprising adding a diluting gasinto the reactant gas.
 14. The method according to claim 13, wherein thediluting gas comprises an inert gas.
 15. The method according to claim13, wherein a volumetric flow rate ratio of the diluting gas to NF₃ toNH₃ is within a range of 10:1:1 to 5:1:5.
 16. The method according toclaim 1, wherein the exposed portion is exposed in a trench.